Used every available vehicle at least once to complete any mission/challenge.
You'll need to complete an entire mission/challenge with every car in order to get this trophy.

List of the cars:

Class A:

Jeep Grand Cherokee SRT8
Dodge Viper SRT8
Orchid XGT

Class B:

Dodge Charger SRT8
Dodge Durango Citadel
XNN 233
Dodge Charger 2010 SRT8 4DR

Class C:

Dodge Challenger SRT8
Chrysler 300 SRT8
Skimmer
Dodge Ram 3500 PowerWagon

Class D:

Dodge DAYTONA 1969
FB 84 Drift
Dodge Dart RT
Jeep Liberty Latitude

Class Special:

Jeep Wrangler Unlimited Rubicon


Note: It will not count as a used if you fail in a challenge, e.g. playing in survival challenge and get your self killed/captured before reaching the end of the line/time. Sure you can get a gold medal in the challenge but it will still not count as a used car.

That's why it's better to play a "Speed Check" challenge, since it's easier/faster to complete.